JP2011004217A - Voltage controlled oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a voltage control oscillator which materializes a wide band of an oscillation frequency without deteriorating residual phase noise characteristics.SOLUTION: In the voltage control oscillator equipped with an active electric circuit which oscillates at a desired frequency in accordance with a control voltage from a control voltage application terminal Vc1, the active electric circuit uses a secondary coil L3 which changes an inductance value by the control voltage applied to a primary coil L5 of a transformer TS as a choke coil for maintaining an oscillation. A resonance frequency between the coil L3 and a capacitor C5 changes according to a change of the inductance value of the coil L3. As a result, the control voltage causes a change of reactance characteristics of the active electric circuit as well as a change of the reactance characteristics by a varicap diode D1 of a resonance circuit.

Description

  The present invention relates to a voltage controlled oscillator used for a radio device or the like, and more particularly to a voltage controlled oscillator that realizes a wide band without degrading residual phase noise characteristics as much as possible.

  As a conventional voltage-controlled oscillator, for example, a Colpitts type oscillation circuit is known. As shown in Patent Document 1, the resonance frequency of a resonance circuit that is an inductive component is obtained by changing the capacitance of a varicap diode by a control voltage. The oscillation frequency is widened by changing the frequency. Hereinafter, the configuration of the conventional voltage controlled oscillator Y will be described in detail with reference to FIG. FIG. 3 is a circuit diagram of a conventional voltage controlled oscillator Y.

  The resonance circuit is composed of a resistor R1, a coil L1, a varicap diode D1, capacitors C1 to C3, a coil L2, and a capacitor C4, and an active circuit is a transistor (N channel JFET) T, capacitors C5 to C8, coils L3 and L4. And resistor R2.

  A control voltage application terminal Vc1 is connected to the cathode of the varicap diode D1 via a resistor R1 and a coil L1 connected in series, and the anode of the varicap diode D1 is grounded. The cathode of the varicap diode D1 is grounded via the capacitor C1 and is connected to one end of the coil L2 via the capacitor C2. One end of a capacitor C3 is connected between the capacitor C2 and the coil L2, and the other end of the capacitor C3 is grounded. The other end of the coil L2 is grounded via a capacitor C4 and is connected to the drain of the transistor T which is an active circuit.

  In addition to the above-described resonance circuit, a parallel resonance circuit including a capacitor C5 connected in parallel to the power supply voltage Vdd and a coil L3 that is a choke coil is connected to the drain of the transistor T. One end of the parallel resonance circuit on the transistor T side and the source of the transistor T are connected via a capacitor C6.

  The source of the transistor T is grounded via a capacitor C7, grounded via a coil L4 and a resistor R2, and connected to the output terminal OUTPUT via a capacitor C8. The gate of the transistor T is grounded. The

  In this voltage controlled oscillator Y, the resonance circuit becomes inductive near the oscillation frequency of the voltage controlled oscillator Y, the active circuit becomes capacitive, and oscillates when matching is established between the resonance circuit and the active circuit. FIG. 4 shows frequency characteristics of reactance of the active circuit and the resonance circuit in the conventional voltage controlled oscillator Y.

  A1 shown in FIG. 4 is a reactance characteristic of the resonance circuit when the minimum control voltage V1 is applied to the varicap diode D1 from the control voltage application terminal Vc1, and A2 is a variable of the maximum control voltage V2 from the control voltage application terminal Vc1. It is a reactance characteristic of the resonance circuit when it is applied to the cap diode D1. B1 is the reactance characteristic of the active circuit.

  The oscillating range of the voltage controlled oscillator Y is from the frequency F1 when the lowest control voltage V1 is applied to the frequency F2 when the highest control voltage V2 is applied. At the oscillation frequency of the voltage controlled oscillator Y shown in FIG. 4, the absolute value of the reactance (P1 or P3) of the resonant circuit showing inductivity and the reactance (P2 or P4) of the active circuit showing capacitive are almost the same. In addition, the connection portion between the resonant circuit and the active circuit is in a state of matching due to a complex conjugate relationship.

JP 2008-042758 A

  In the conventional voltage controlled oscillator Y, when it is intended to realize a wide band by optimizing a constant, it is conceivable to shift the reactance characteristic A2 of the resonance circuit to the high frequency side. However, if the reactance characteristic A2 is shifted too much to the high frequency side, the C / N ratio of the voltage controlled oscillator Y deteriorates, so that the residual phase noise characteristic deteriorates and the application of a wider band for which the phase shift must be kept low. It becomes difficult.

  In addition, if an attempt is made to forcibly widen the band, the negative resistance value in the impedance characteristic of the active circuit may become small, and oscillation may stop in a region where the Q value of the resonance circuit rapidly deteriorates.

  Further, although a wide band system for switching the varicap diode and the entire resonance circuit is conceivable, the redundancy of the circuit becomes very large, and the mounting area and the component cost are wasted.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a voltage controlled oscillator that realizes a wide band without deteriorating residual phase noise characteristics.

  The voltage controlled oscillator of the present invention is a voltage controlled oscillator including an active circuit that oscillates at a desired frequency in accordance with a control voltage, and the active circuit has an inductance by the control voltage applied to a primary coil of a transformer. The secondary coil of the transformer whose value changes is used as a choke coil for maintaining oscillation.

  According to the present invention, since the reactance characteristic of the active circuit can be changed, the oscillation frequency can be widened without deteriorating the residual phase noise characteristic.

1 is a circuit diagram of a voltage controlled oscillator X according to an embodiment of the present invention. It is a figure which shows the reactance versus frequency characteristic of the voltage controlled oscillator X which concerns on embodiment of this invention. FIG. 6 is a circuit diagram of a voltage controlled oscillator Y according to a conventional embodiment. It is a figure which shows the reactance versus frequency characteristic of the voltage controlled oscillator Y which concerns on the conventional embodiment.

  In the conventional voltage controlled oscillator Y shown in FIG. 3, the resonance frequency of the capacitor C5 and the coil L3, which is a choke coil, becomes a large factor, which affects the reactance characteristics of the active circuit. The voltage controlled oscillator of this embodiment uses the secondary side of the transformer as the choke coil of the active circuit, and controls the inductance value of the secondary side coil of the transformer by the control voltage (DC voltage) applied to the primary side coil of the transformer. Thus, the reactance characteristic of the active circuit can be shifted on the frequency axis. As a result, even when the variable range of the reactance characteristic of the resonance circuit is narrow, the oscillation frequency can be widened without deteriorating the residual phase noise characteristic. Hereinafter, the voltage controlled oscillator of this embodiment will be described in detail with reference to the drawings. The same components as those of the conventional voltage controlled oscillator Y are denoted by the same reference numerals.

  In the voltage controlled oscillator X of this embodiment shown in FIG. 1, as in the case of the conventional voltage controlled oscillator Y, a resonance composed of a resistor R1, a coil L1, a varicap diode D1, capacitors C1 to C3, a coil L2, and a capacitor C4. An output signal from the circuit is input between the gate and drain of the transistor T which is an active circuit. This resonance circuit can output to the active circuit an output signal whose resonance frequency is changed by changing the capacitance of the varicap diode D1 according to the control voltage applied from the control voltage application terminal Vc1.

  The active circuit of the voltage controlled oscillator X includes a transistor T, capacitors C5 to C8, a coil L4, a resistor R2, a transformer TS, a resistor R3, and an operational amplifier OP. In the active circuit of this embodiment, the coil L3, which is a choke coil for maintaining the oscillation of the active circuit in the conventional control voltage oscillator Y, is used as the secondary coil of the transformer TS. In the configuration of the active circuit, the transistor T, the capacitors C5 to C8, the coils L3 and L4, and the resistor R2 are the same as those of the conventional voltage controlled oscillator Y, and thus description thereof is omitted.

  A voltage corresponding to the control voltage application terminal Vc1 is applied to the primary side of the transformer TS. The voltage applied to the primary side of the transformer TS may be the same voltage as the control voltage application terminal Vc1, or may be a proportional voltage, and the inductance value of the secondary coil L3 of the transformer TS is set to a desired value. Any voltage that can be changed to a value may be used. However, in order to prevent the impedance on the primary side of the transformer TS from affecting the input of the control voltage, the primary side of the transformer TS and the control voltage application terminal Vc1 are connected via a buffer amplifier composed of an operational amplifier OP. Is connected.

  Specifically, one end of the primary coil L5 of the transformer TS is grounded, and the other end is connected to the output terminal of the operational amplifier OP via the resistor R3. A control voltage sign application terminal Vc1 is connected to the non-inverting input terminal of the operational amplifier OP, and the output terminal of the operational amplifier OP is connected to the inverting input terminal.

  The inductance value of the secondary coil L3 of the transformer TS is controlled by the control voltage applied to the primary coil L5 of the transformer TS. This is because, when the control voltage applied to the primary side of the transformer TS changes, the magnetic flux generated in the iron core of the transformer TS changes, so that the inductance value of the secondary coil L3 of the transformer TS changes. Since the resonance frequency of the coil L3 and the capacitor C5 changes due to the change in the inductance value of the coil L3, the reactance characteristic of the active circuit changes. That is, the reactance characteristic of the active circuit can be shifted on the frequency axis by the control voltage from the control voltage application terminal Vc1.

  FIG. 2 shows frequency characteristics of reactance of the active circuit and the resonance circuit in the voltage controlled oscillator X of the present embodiment.

  In FIG. 2, A1 is the reactance characteristic of the resonance circuit when the lowest control voltage V1 is applied to the varicap diode D1 from the control voltage application terminal Vc1, and A2 is the variable of the highest control voltage V2 from the control voltage application terminal Vc1. The reactance characteristic of the resonance circuit when applied to the cap diode D1 is the same as the reactance characteristic of the resonance circuit of the conventional voltage-controlled oscillator Y shown in FIG. B3 is the reactance characteristic of the active circuit when the lowest control voltage V1 is applied to the primary side of the transformer TS from the control voltage application terminal Vc1, and B4 is the primary control voltage V2 from the control voltage application terminal Vc1 to the primary of the transformer TS. It is the reactance characteristic of an active circuit when it applies to the side.

  In the present embodiment, since the reactance characteristic of the active circuit can be shifted on the frequency axis, the reactance (P5 or P7) of the resonance circuit exhibiting inductivity and the capacitance are compared with the case where the reactance characteristic of the active circuit is fixed. The frequency band that can be matched with the reactance (P6 or P8) of the active circuit is wide (P5 and P6 indicate that the absolute values of the reactance values are substantially equal. The same applies to P7 and P8). Therefore, the oscillatable range of the voltage controlled oscillator X is from the frequency F3 when the lowest control voltage V1 is applied to the frequency F4 when the highest control voltage V2 is applied, and the bandwidth can be increased as compared with the prior art.

  In addition, when realizing a wider bandwidth on the high frequency side, the reactance characteristic of the active circuit is shifted to the high frequency side, so that the frequency band that can be matched on the high frequency side is widened. There is no need to shift, and the residual phase noise characteristics do not deteriorate. Therefore, even if the variable range of the reactance characteristic of the resonance circuit is narrow, stable oscillation can be maintained while realizing a wide band by shifting the reactance characteristic of the active circuit.

  In this embodiment, in order to change the reactance characteristic of the active circuit, a coil L3 having a variable inductance value and a capacitor C5 having a fixed capacitance value are connected, and the resonance frequency of the coil L3 and the capacitor C5 is changed. Although it has been described that it is realized, it may be configured such that the capacitance value of the capacitor C5 can be changed by a varicap diode or the like. By making the inductance value of the coil L3 and the capacitance value of the capacitor C5 variable, the reactance characteristic of the active circuit can be adjusted more flexibly. If adjustment of the resonance frequency by the coil L3 and the capacitor C5 is not necessary, the capacitor C5 may not be connected. This is because the reactance characteristic of the active circuit changes as the inductance value of the coil L3, which is the secondary side of the transformer, changes according to the control voltage applied to the primary side of the transformer.

  In this embodiment, the operational amplifier OP is interposed between the primary side of the transformer TS and the control voltage application terminal Vc1 so that the impedance on the primary side of the transformer TS does not affect the input of the control voltage. Instead of the operational amplifier OP, for example, a MOS transistor may be interposed. Specifically, the gate of the MOS transistor is connected to the control voltage application terminal Vc1, the source of the MOS transistor is connected to the power supply voltage Vdd, and the drain of the MOS transistor is grounded through a resistor. Then, one end of the resistor R3 not connected to the transformer TS may be connected to a connection line between the drain of the MOS transistor and the resistor.

In summary, the present invention provides a voltage controlled oscillator including an active circuit that oscillates at a desired frequency in accordance with a control voltage, the active circuit being controlled by the control voltage applied to a primary coil of a transformer. The secondary coil of the transformer whose inductance value changes is used as a choke coil for maintaining oscillation.
A capacitor may be connected to the secondary coil.
An operational amplifier may be connected between the primary coil and a terminal to which the control voltage is applied.

  The present invention is not limited to the above-described embodiment, and it goes without saying that various changes can be made without departing from the spirit of the present invention.

C1 to C8 ... capacitors L1 to L5 ... coil D1 ... varicap diodes R1 to R3 ... resistance T ... (field effect type) transistor TS ... transformer OP ... operational amplifier X , Y ... Voltage controlled oscillator

Claims (1)

A voltage controlled oscillator including an active circuit that oscillates at a desired frequency according to a control voltage,
The active circuit uses a secondary coil of the transformer whose inductance value changes according to the control voltage applied to the primary coil of the transformer as a choke coil for maintaining oscillation.

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